1. Field of Invention
The present invention relates to phase shift mask (PSM). More particularly, the present invention relates to half-tone phase shift mask.
2. Description of Related Art
Currently, photolithography plays an essential role in semiconductor fabrication. For example, processes for patterning a structure or doping a region in a wafer always require several photolithography processes. In a photolithography process, exposure resolution and depth of focus (DOF) are two important parameters that determine photolithography quality.
A binary mask, which is made of quartz having chrome pattern coated thereon and is used for design rule above 0.18 .mu.m in semiconductor fabrication, can obtain a pattern with high quality on the photoresist layer. However, when the semiconductor structure is required to be under 0.18 .mu.m in size, diffraction of light causes serious problems due to the reduced size of the hole and line on the mask. In the photolithography process, especially, for forming a poly gate in the logic circuitry region of dynamic random access memory (DRAM), a desired pattern such as sole poly line is hard to obtain on the wafer because of the diffraction of light. As a result, the incorrect pattern causes a failure of a subsequent etching process of poly line and yield of the product is thus reduced.